Hydrocarbons that are gaseous at room temperature such as 20° C., are often transported by tanker as LNG (liquified natural gas) at −160° C. and atmospheric pressure. Other cold forms during transport are hydrates (gas entrapped in ice) and cooled CNG (compressed natural gas that has been cooled well below 0° C. to reduce the pressure required to keep it liquid). At the tanker's destination, the LNG (or other cold gas) may be offloaded, heated and pressurized, and carried by pipeline to an onshore station for distribution (or possibly for use as by a power plant at the onshore station).
Proposed prior art offloading and regas/injection systems (for heating and pressuring LNG) include a fixed platform extending up from the sea floor to a height above the sea surface and containing facilities that heat and pump the cold hydrocarbons and containing crew facilities (beds, toilet, food storage, etc.). The heating is sufficient to transform LNG into gas that is warm enough (usually at least 0° C.) to avoid ice formations around noncryogenic hoses and pipes that carry the gas. The platform also carries a pump system that pumps the gas to a high enough pressure to pump it along a sea floor pipeline to an onshore station, and/or to a cavern and maintain a high pressure in the cavern so gas can flow therefrom to an onshore station. A platform that is large enough to carry such gas heating and pumping systems can be expensive even in shallow waters.
It is possible to greatly lower costs by the use of a floating weathervaning structure such as a barge with a turret near the bow, that is moored by catenary chains to the sea floor, to carry the regas and pressurizing equipment and crew quarters, and to moor the tanker. However, in shallow depths (e.g. less than about 70 meters), drifting of the vessel tends to lift the entire length of chain off the sea floor. This can result in a sudden increase in chain tension rather that a gradual increase that is required. A system of minimum cost, for mooring a tanker, offloading LNG from the tanker, heating and pressuring the LNG, accommodating any crew, and flowing the gaseous hydrocarbons to an onshore station, in a sea location of shallow depth, would be of value.